1. Field of the Invention
The present invention relates to an optical fiber network that provides both broadband switched video services and narrowband telephony signals over a single fiber using a single optical carrier, and more particularly, to an optical network unit located in subscriber neighborhoods for the purpose of converting optical signals to electrical signals and for providing metallic drops to the various subscribers.
2. Description of the Prior Art
The rapid proliferation of optical fiber telecommunications systems has made possible the provision of broadband services to individual subscribers on a relatively universal basis. Such broadband services often include data transmission; however, there is a broader market for the distribution of video signals over the telecommunications network.
The provision of such video services has long been desired; however, the previously proposed systems have all been subject to various deficiencies which have prevented their commercial acceptance. Video signals may be broadcast to all subscribers over optical fibers; however, this severely limits the programming selection and the number of channels that may be available to each subscriber. A switched video architecture allows for the provision of significantly more programming options and control of distribution only to authorized subscribers.
The concept of switched video transmission systems has been proposed in the past; however, most proposals have had undesirable features. Most proposed switched video architectures require the use of a second optical fiber to distribute the broadband services or, as an alternative, the use of a wavelength division multiplexing system. Such systems do not offer a truly integrated architecture, wherein a single fiber distributes both narrowband and broadband signals and the systems are not integrated with respect to both control and cost effective utilization of common electronics.
An article entitled: "A Future Switched Video System" by John R. Gunter, IEEE LCS Magazine, February, 1990, at page 66 and following, describes the desirability of providing video services over the telecommunications network. Another article entitled: "A High-Quality Switched FM Video System" by David E. Robinson and David Grubb, III, IEEE LCS Magazine, also published February, 1990, at page 53 and following describes a proposed system architecture wherein the various video channels are frequency multiplexed onto a carrier; however, the carrier uses wavelength division multiplexing for upstream and downstream transmissions.
Other articles describing the simultaneous transmission of narrowband and broadband signals are as follows: "A Hybrid Lightwave Transmission System for Subcarrier Multiplexed Video and Digital B-ISDN Services in the Local Loop", by Charles N. Lo, Journal of Lightwave Technology, Vol. 7, No. 11, November 1989, pp. 1839-1848; and "Fiber Optic Analog-Digital Hybrid Signal Transmission Employing Frequency Modulation", by K. Sato et al, IEEE Transactions on Communications, Vol. COM-33, No. 5, May 1985, pp. 433-441.
In these networks, the Optical Network Unit (ONU) receives power from a local power cable, and both broadband switched video signals and narrowband telephony signals delivered over a single optical fiber. The ONU serves as the distribution unit, separating and converting the arriving signals into independent electrical signals, which are then delivered as video signals over co-axial cables and telephony signals. over twisted pair cables to nearby subscribers, residences.
Considerable heat builds up in the sealed, moisture proof and air-tight ONU enclosures, which may be underground, mounted above ground on walls, poles or pedestals, or strand-mounted, suspended from the overhead cable. To avoid overheating, means are needed to conduct heat to the outer surfaces of the ONU enclosures, where it can be dissipated to the surrounding ambient atmosphere.